Add userAgentAllowsProtocol static method (#221)

Author: timcappalli

index.html

@@ -462,6 +462,7 @@ <h2>
     interface DigitalCredential : Credential {
       readonly attribute DOMString protocol;
       [SameObject] readonly attribute object data;
+      static boolean userAgentAllowsProtocol(DOMString protocol);
     };
     </pre>
     <p>
@@ -488,6 +489,44 @@ <h2>
       Integration with Credential Management API
     </h2>
     <aside class="issue" data-number="65"></aside>
+    <h3>
+      The <dfn data-dfn-for="DigitalCredential">userAgentAllowsProtocol()</dfn>
+      method
+    </h3>
+    <p>
+      The {{DigitalCredential/userAgentAllowsProtocol()}} method allows digital
+      credential [=verifiers=] to determine which [=digital credential/exchange
+      protocols=] and [=digital credential/issuance protocols=] the user agent
+      allows.
+    </p>
+    <p class="note">
+      This method does not convey [=digital credential/exchange protocol=] or
+      [=digital credential/issuance protocol=] support in the underlying
+      OS/platform.
+    </p>
+    <p>
+      User agents MUST NOT vary the response value based on any information
+      about availability of hardware, presence or configuration of software,
+      wallets, credential providers, or digital credentials, or user
+      configuration or preferences. If the response value varied, the user
+      agent would introduce risks both of fingerprinting and of silently
+      revealing other details about user behavior or configuration. The
+      response value SHOULD vary only by user agent major version and indicate
+      whether the browser supports distributing requests with that protocol to
+      underlying platform or provider.
+    </p>
+    <p>
+      When this method is invoked, the user agent MUST execute the following
+      algorithm:
+    </p>
+    <ol class="algorithm">
+      <li>If |protocol| is not a [=digital credential/protocol identifier=],
+      return `false`.
+      </li>
+      <li>Return `true` if the user agent allows |protocol|, otherwise return
+      `false`.
+      </li>
+    </ol>
     <h3>
       [[\DiscoverFromExternalSource]](origin, options, sameOriginWithAncestors)
       internal method